1. Field of the Invention
The present invention relates in general to automatic fine tuning (AFT) of a television (TV) receiver employing a voltage synthesizer (VS), and more particularly to an apparatus and a method for extending a pulling range of the automatic fine tuning of the TV receiver, thereby to prevent a shortage in the pulling range of the automatic fine tuning when a TV channel is switched or the TV receiver is powered on and a detuning due to a variation in characteristics of components at a high or low temperature.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of a conventional automatic fine tuning system for a TV receiver employing a video synthesizer. As shown in this drawing, the conventional automatic fine tuning system comprises a tuner 24 for tuning a TV broadcasting signal received through an antenna ANT to a specified channel corresponding to a tuning voltage from a microcomputer 23 and outputting an intermediate frequency (IF) signal of the tuned TV broadcasting signal, and an IF processor 25 for processing the IF signal of the tuned TV broadcasting signal from the tuner 24 to output an automatic fine tuning voltage V.sub.AFT and a synchronous signal to the microcomputer 23 and a video signal of the tuned TV broadcasting signal to a video processor 26.
The video processor 26 is adapted to process the video signal from the IF processor 25 to output red (R), green (G) and blue (B) color signals.
The conventional automatic fine tuning system also comprises a key matrix unit 21 for outputting an electrical signal corresponding to a pushed key thereon to the microcomputer 23, and a remote receiver 22 for receiving a remote signal from a remote controller (not shown), converting the received remote signal into an electrical signal and outputting the converted electrical signal to the microcomputer 23.
The microcomputer 23 is adapted to check input of the automatic fine tuning voltage V.sub.AFT and the synchronous signal from the IF processor 25. Also, the microcomputer 23 generates the tuning voltage in response to tuning data from the remote receiver 22 or the key matrix unit 21 and outputs the generated tuning voltage to the tuner 24, thereby allowing the tuner 24 to tune the channel finely.
Also, the conventional automatic fine tuning system comprises an on-screen display (OSD) character generator 27 for generating a caption character signal addressed by the microcomputer 23 when the corresponding channel is tuned by the tuner 24, and a video synthesizer 28 for synthesizing the caption character signal from the OSD character generator 27 and the R, G and B color signals from the video processor 26 and outputting the resultant signal to a color picture tube (CPT) 29.
The operation of the conventional automatic fine tuning system with the abovementioned construction will hereinafter be described with reference to FIGS. 2A to 3. FIGS. 2A and 2B are waveform diagrams of the automatic fine tuning voltage V.sub.AFT and the synchronous signal from the IF processor 25 in FIG. 1, respectively, and FIG. 3 is a flowchart illustrating a conventional method of controlling the automatic fine tuning voltage V.sub.AFT from the IF processor 25 in FIG. 1.
At the first step S31 of FIG. 3, in the case where a desired key on the remote controller, not shown, is pushed by the user to power on the TV receiver or switch the TV channel, the corresponding remote signal, perhaps an infrared-ray signal, is generated from the remote controller and then received by the remote receiver 22. The remote receiver 22 converts the received remote signal into the electrical signal and outputs the converted electrical signal as the tuning data to the microcomputer 23. Alternatively, at the first step S31, when a desired key on the key matrix unit 21 is pushed by the user to power on the TV receiver or switch the TV channel, the corresponding electrical signal from the key matrix unit 21 is applied as the tuning data to the microcomputer 23.
Upon receiving the tuning data from the remote receiver 22 or the key matrix unit 21, the microcomputer 23 outputs an initial tuning voltage as shown in FIG. 2A to the tuner 24 to select an initial channel band, at the second step S32. As shown in FIG. 2A, the initial tuning voltage from the microcomputer 23 corresponds to a center frequency fo.
Then, the tuner 24 tunes the TV broadcasting signal received through the antenna ANT to the specified channel corresponding to the initial tuning voltage from the microcomputer 23 and outputs the IF signal of the tuned TV broadcasting signal to the IF processor 25. The IF processor 25 processes the IF signal of the tuned TV broadcasting signal from the tuner 24, so as to output the video signal of the tuned TV broadcasting signal to the video processor 26. Also, the IF processor 25 outputs the automatic fine tuning voltage V.sub.AFT and the synchronous signal to automatic fine tuning and identification terminals AFT and ID of the microcomputer 23, respectively.
At the third step S33, the operation of the microcomputer 23 is delayed for a predetermined time period (for example, 300 ms) as shown in FIG. 2B from the power-on till a normal state of the TV receiver. After the lapse of the predetermined time period, the microcomputer 23 checks the input of the synchronous signal from the IF processor 25 to its identification terminal ID at the fourth step S34.
If it is checked at the fourth step S34 that the synchronous signal from the IF processor 25 is not inputted by the identification terminal ID, the microcomputer 23 returns to the second step S32 to continue to output the initial tuning voltage corresponding to the initial center frequency fo to the tuner 24. On the contrary, if it is checked at the fourth step S34 that the synchronous signal from the IF processor 25 is inputted by the identification terminal ID, the microcomputer 23 proceeds to the fifth step S35.
At the fifth step S35, the microcomputer 23 compares the automatic fine tuning voltage V.sub.AFT which is fed back from the IF processor 25 to its automatic fine tuning terminal AFT with first and second predetermined reference voltages having a desired range of 2.0-3.0 V. If the automatic fine tuning voltage V.sub.AFT, initially stored, corresponding to the initial center frequency fo is within the desired range of 2.0-3.0 V as shown in FIG. 2A in accordance with the compared result, namely, the normal state, the microcomputer 23 returns to the third step S33 to perform it again for the predetermined time period.
On the contrary, if the automatic fine tuning voltage V.sub.AFT corresponding to the initial center frequency fo is not within the desired range of 2.0-3.0 V as a result of the comparison, the microcomputer 23 performs the sixth step S36 and then returns to the third step S33.
At the sixth step S36, if the automatic fine tuning voltage V.sub.AFT is lower than the first predetermined reference voltage of 2.0 V, the microcomputer 23 decreases the tuning voltage to the tuner 24. If the automatic fine tuning voltage V.sub.AFT is higher than the second predetermined reference voltage of 3.0 V, the microcomputer 23 increases the tuning voltage to the tuner 24.
Therefore, the automatic fine tuning voltage V.sub.AFT red back from the IF processor 25 is connected to always have the desired range of 2.0-3.0 V, thereby allowing the tuner 24 to tune the channel finely.
In detail, if the automatic fine tuning voltage V.sub.AFT is lower, at a point b in FIG. 2A, than the first predetermined reference voltage of 2.0 V, the microcomputer 23 outputs a new tuning voltage lower than the previous one to the tuner 24. If the automatic fine tuning voltage V.sub.AFT is higher, at a point a in FIG. 2A, than the second predetermined reference voltage of 3.0 V, the microcomputer 23 outputs a new tuning voltage higher than the previous one to the tuner 24. In this manner, the automatic fine tuning voltage is corrected.
By the way, when the synchronous signal is not detected or the automatic fine tuning voltage V.sub.AFT is not within the desired range of 2.0-3.0 V although the above operation is repeatedly performed at least once, the microcomputer 23 considers the corresponding channel to have no broadcasting signal. Hence, the microcomputer 23 outputs the tuning voltage corresponding to the center frequency to the tuner 24 and then stops the automatic fine tuning operation.
On the other hand, the OSD character generator 27 generates the caption character signal addressed by the microcomputer 23 when the corresponding channel is tuned by the tuner 24. The caption character signal from the OSD character generator 27 is applied to the video synthesizer 28, which also receives the R, G and B color signals from the video processor 26. Then, the video synthesizer 28 synthesizes the received caption character signal and R, G and B color signals and outputs the resultant signal to the CPT 29.
Noticeably, in the above-mentioned conventional automatic fine tuning system for the TV receiver employing the video synthesizer, if the automatic fine tuning voltage is detected in an audio band to be lower than 2.0 V as shown by a point d in FIG. 2A because of temperature characteristics of the tuner 24 and the IF processor 25, file microcomputer 23 outputs a new tuning voltage lower than the previous one to the tuner 24, thereby to correct the automatic fine tuning voltage to have the desired range of 2.0-3.0 V. Also, if the automatic fine tuning voltage is detected in the audio band to be higher than 3.0 V as shown by a point c in FIG. 2A, the microcomputer 23 outputs a new tuning voltage higher than the previous one to the tuner 24. As a result, the automatic fine tuning voltage is corrected to have the desired range of 2.0-3.0 V. In this connection, the above-mentioned conventional automatic fine tuning system has a disadvantage in that an asynchronous picture is displayed on the CPT 29 because the channel is tuned under the condition that the center frequency fo is detuned upwardly by .DELTA.f (2 MHz). Also, the display picture is flickered on the CPT 29 for the time required in automatically tuning the channel finely.